This invention relates to heat recovery and more particularly to a method and apparatus for recovering said heat from waste gas streams.
One current problem with many industrial processes is the safe and economical disposal of waste water and other waste fluid streams generated by the process. This problem is complicated by the fact that many of these streams are contaminated with one or more dissolved or suspended components which are present in amounts above those allowed by present day federal and/or state regulations. Where such contamination is present, the costs involved with either transporting large amounts of a relatively dilute solution to a treatment facility or of internally concentrating said solution so that the volume of material which must be handled is substantially reduced, are often a significant part of the total costs of the manufacturing process from which the waste fluid streams emanated. Further, many of these contaminants have a substantial economic value which, if they could be economically recovered, their recovery could be used to offset total costs.
Accordingly, what is needed is a low cost process which utilizes a source of energy either to evaporate volatile agents such as industrial solvents for recovery, or to concentrate the waste fluid streams so that conventional precipitation, filtration and/or other methods of treatment and disposal can be more efficiently conducted.
One such source of energy is the hot waste flue gas, exhaust air, and the like which are generated from industrial/commercial furnaces, ovens, incinerators, and similar devices and which often contain substantial quantities of heat energy. These gases are commonly discharged through convenient duct work (stacks) leading to the environment. While these "stack" gases may have temperatures as high as 1500.degree. F. or more, this heat energy is frequently lost and abandoned due to the complexity, cost, and low efficiency of many present day methods for recovering and using their heat energy. Furthermore, many of these gas streams are themselves contaminated with large quantities of water vapor and carbon dioxide, as well as smaller amounts of carbon monoxide, unburned hydrocarbons, fly ash, acidic sulfur and nitrogen oxides, hydrogen chloride and other vaporous or solid entrained species, all of which act to render such streams obnoxious, corrosive or abrasive. As a result, direct use of such gas streams is generally avoided as a heat source even in potential applications where the above noted problems could be tolerated.
In part, this avoidance can be particularly attributed to the large amount of water vapor frequently present in these gases. If the stack gases are piped directly from their point of origin to some application for recovery and use of their heat energy, the resultant cooling could easily cause at least some of this vapor to condense, resulting in a significant quantity of liquid water flowing back to the source of the heat energy. Such an event could seriously damage furnaces, ovens, or other equipment used to generate the heat. Indeed, in situations where molten metals are involved, contact with even a small amount of liquid water could result in a serious explosion. Further, if the gas is used to heat a quantity of liquid, any leakage from the vessel containing such liquid could lead to the same undesirable result.
As a result, most conventional schemes to recover and utilize heat values from stack gases rely on the use of heat exchangers having a pressurized heat exchange fluid to transfer heat from the hot gases for use in another process. Since such a system often requires that the heat transfer fluid be circulated through a second heat exchanger located in contact with the process at the point where the heat energy is to be utilized it is inherently inefficient owing to the multiple heat transfers that are needed for the process to operate. Further, the aforementioned problems with free flowing condensate and/or system leakage are not really alleviated to any great degree and, in fact, may be aggravated since a second, pressurized heat transfer fluid is now involved.
In view of these considerations, the routine utilization of waste stack gas as a source of heat energy for industrial use is often ignored or dismissed as impractical. Consequently, industries that deal in the refining, casting, and working of metals such as steel, aluminum, zinc, copper, and the like, frequently discharge substantial quantities of waste heat energy in stack gases with little regard being given to waste heat recovery schemes even for the evaporative purification of waste water which has been contaminated by their basic processes.
Similarly, many large incinerators are operated for the purpose of reducing the volume of commercial and domestic wastes through combustion. Here, too, little regard is usually given to utilizing the heat generated by such operations for the evaporative purification of system cooling water or the drying of waste sludges either for easier transport and/or resource recovery.
Since the costs of treating waste water streams to meet EPA, RICRA and state requirements for the contaminants therein often have a signigicant impact on the competitiveness of many industrial processes, the advantages of either economically recovering the water or concentrating the waste for treatment are obvious. Further, in view of the number of such sources of waste heat, the magnitude of the annual thermal energy loss resulting by such discharge must be of truly staggering proportions. In addition, the aggregate heat content in such discharge, especially in urban areas, has the capacity to cause severe environmental perturbations. Clearly, advances in the techniques for stack gas heat recovery are required so that this energy resource can be viably applied for beneficial purposes.